24 June 2021

Several years of detective work in the laboratory and sitting at a scanning electron microscope have increased our knowledge of stainless steel. The results may lead to new materials to be used in the boilers of power plants, with higher efficiency, lower emissions and a smaller impact on the climate.

Cracks in austenitic stainless steel.
Cracks in austenitic stainless steel.

Interesting combination

Hugo Wärner.Hugo Wärner.

That’s exactly how Hugo Wärner describes it: “detective work”. In his doctoral thesis, High Temperature Fatigue Behaviour of Austenitic Stainless Steel: Microstructural Evolution during Dwell-Fatigue and Thermomechanical Fatigue, he investigates the properties of what are known as austenitic stainless steels at high temperature, high pressure and frequent changes in loading.

Behind the thesis lie many hours of work with mechanical samples in the laboratory, during which test rods were placed under load until they either cracked or failed completely, followed by investigation of the fracture surfaces in the scanning electron microscope. Then followed analysis of the results and finally writing summary reports. The thesis comprises six scientific articles.

“It’s a combination of practical and theoretical work, which I find extremely satisfying. I started the investigation of microstructure without knowing what I would find. The first step was to see what happens to the material, and then to combine the observations with a hypothesis and previous results”, says Hugo Wärner.

Clear limitations

Testing equipment.Testing equipment.

The temperature most usually used today in the boilers of power plants is between 500 and 600 degrees, but higher temperatures and pressures will be needed if we are to increase efficiency and reduce emissions. At the same time, the requirement that the plants operate cyclically is increasing. This introduces many start-ups and shut-downs, when the power plants are used as back-up for energy sources that depend on the weather, such as wind power and solar energy.

A fundamental question for the scientists is how austenitic stainless steels perform when placed under heavier loading, and whether they can be used also in the boilers of the future. Stainless steel is considerably cheaper than nickel-based superalloys, which are otherwise the main alternative for operations of this type.

The results that Hugo Wärner presents in the thesis, where he has subjected test rods to temperatures up to 800 degrees, show that stainless steels have clear limitations. A process known as creep (slow deformation under mechanical load), a combination of fatigue and creep, and the formation of cracks due to oxidation all combine to shorten the lifetime of the material. In addition, ageing (in which the microstructural configuration of the metal changes with time at high temperature) reduces the resistance of the material to damage mechanisms.

On the other hand, highly alloyed austenitic stainless steels have better high-tempexrature properties than the materials traditionally used in boilers, due to a more advanced alloying process.

Can be alternative

Mjölby CHP plant.Mjölby CHP plant. Photo credit Jarl Asklund

So to sum up – can these materials be used in tough operating conditions? And can they replace nickel-based superalloys?

“I suggest that this is a useful intermediate stage. The nickel-based superalloys have better properties, but before we reach conditions in which they must be used, austenitic alloys can be a useful alternative during a transition period. But it’s difficult to predict how long that period can last”, says Hugo Wärner.

Some of the results presented in the thesis were expected: others were somewhat surprising. While most samples behaved as expected, one material, in particular, showed unexpected behaviour. The reasons for this will be the subject of future research.

“What I have done, more than anything else, is to show how different materials behave in different conditions. The next step will be to explore the measures to take to counteract the problems we have found. I give some suggestions in the thesis, but it focusses rather more on describing various occurrences”, says Hugo Wärner.

The research in the thesis is part of the research projects Influence of high-temperature environments on the mechanical behaviours of high-temperature austenitic stainless steels and Heavy section austenitic stainless steel for the future header and piping material in high-efficient biomass-fired power plants, financed by the Swedish Energy Agency and carried out in collaboration with Sandvik. Independently of future applications, one important aim is to increase theoretical knowledge of steel as a material.

Footnote: Austenitic steel is steel that retains a particular crystal structure, austenite, at room temperature due to the addition of certain alloying compounds. These can be carbon, manganese and nickel. The steel is highly corrosion-resistant and is easy to shape and weld. (NE)

Translated by George Farrants

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